Assigning reference designators for the schematic instances is a very vital part of the entire PCB flow. This can sometimes become very cumbersome, and in some cases users allocate a major portion of their time and effort to get the assignments correct and optimized.
Annotation is the automated process of assigning reference designators in Allegro Design Entry CIS, also known as OrCAD Capture. The following AppNote clarifies the fundamentals of the Instance and occurrence modes of annotation in a Capture based design. It explains various aspects of annotation and simplifies the concept behind Instance and Occurrence modes.
What are Instance and Occurrence Modes?
These two modes essentially determine how a design is annotated. The Annotate dialog, as shown in Fig.2, provides the option to annotate a design in Instance or Occurrence modes. The recommended mode of annotation is determined based on the conditions specified in the following table:
Table.1 - Recommended annotation modes
Fig.2 - Annotate Dialog Box
The property editor for any part in a Capture design has a white column and one or more yellow columns. The white column is the instance column and yellow columns are occurrence columns.
Flat and Simple Hierarchical Design
With the above explanation, we can deduce that no part contains duplicate occurrence in a flat or simple hierarchical design. The property editor contains one white and one yellow column for every part and both contain the same value for all the properties. By default, the yellow column is hidden for an INSTANCE mode design. You can click the plus sign to expand the yellow column.
Fig. 3 - Property Editor of a Part in a Flat/Simple-Hierarchical Design
For Complex Designs
The property editor includes a yellow column for each occurrence of a part. If a design contains 3 duplicate hierarchical blocks, for all the parts within that hierarchical block, the property editor will contain one white and three yellow columns.
The Part Reference of parts in yellow columns (at the Occurrence level) must be unique after correct annotation of the design.
Fig.4 - Property Editor of a Part in a Complex Hierarchical Design
In Fig.4, observe that capacitors have four occurrences in the design. C1 has four occurrences, C1, C5, C9 and C13.
Annotation is the automated process of assigning reference designators to all the parts placed in the design. Under ideal conditions, annotation must be done as shown in Table1.
However, you can select the desired radio button in Fig.2 for any type of design. So, let's understand what exactly happens when the INSTANCE or OCCURRENCE radio buttons are selected.
When a design is annotated in the Instance mode, the part reference is assigned/modified in the white column, representing the instance mode, of the property editor.
As a flat or simple hierarchical design is expected to have the same values in the white and yellow columns, this is the preferred mode of annotation for a flat or simple hierarchical design.
When a design is annotated in the Occurrence mode:
- Mostly, the part reference is assigned/modified in the yellow column representing the Occurrence mode of the property editor.
- At times, the Occurrence value for a property may be picked from the Instance column. Such columns appear striped. In Fig 4a, L1 is striped because it is being picked from instance columns while L3 is not striped because it has been assigned at the occurrence level. (See Fig 5)
Fig 5 - Occurrence mode annotation
Note: As a part will have more than one occurrence in a complex hierarchical design, it is essential that all these occurrences have a unique reference designator in the design. For this, the yellow columns for the parts must have unique reference designator. Therefore, for a complex hierarchical design, the preferred mode of annotation is Occurrence. This ensures that each occurrence gets a unique reference designator.
You can also perform controlled annotation in a multi-page design or a design which contains hierarchical blocks. You can specify the range of reference designator under a hierarchical block or a page. To do this, use the Refdes control required option in the Annotate dialog. Selecting this option gives an additional control to specify range for reference designators as per the hierarchical block or schematic pages.
Fig 6 - options for controlled annotation
For hierarchical designs, you can define a range for each hierarchical block. For flat designs, you can define a range for schematic pages.
Exception in Design Annotation Modes
Sometimes it can be seen that for a flat or simple hierarchical design, the preferred annotation mode is Occurrence. This is the case when any property value has been manually modified in the yellow column (occurrence level). Even adding a space in a property value at the occurrence level will make the preferred mode change from occurrence to instance. In such cases, the preferred mode can be changed using the Accessories > Transfer Occ. Prop. to Instance > Push Occ. Prop into Instance command. Sometimes it can be seen that for a flat or simple hierarchical design, the preferred annotation mode is Occurrence. This is the case when any property value has been manually modified in the yellow column (occurrence level). Even adding a space in a property value at the occurrence level will make the preferred mode change from occurrence to instance. In such cases, the preferred mode can be changed using the Accessories > Transfer Occ. Prop. to Instance > Push Occ. Prop into Instance command.
This will transfer all the yellow column property values (occurrence level properties) to white column (Instance), making both the same and switching the design back to the Instance mode.
Refer the following AppNote for the detailed understanding of these modes in the Capture - Allegro PCB Editor flow.
Click here for the AppNote.
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